1. Field of the Invention
This invention relates to a reversible light sensitive glass which has a high refractive index (n.sub.d value higher than 1.50) and in which the present transmission is rapidly and reversibly changed depending on the radiation of ultraviolet rays and visible light of a short wave length.
2. Description of the Prior Art
Generally, a light shielding glass lens must be made from a reversible light sensitive glass having a property of rapidly responding to light. If the refractive index is increased, not only can the thickness of the glass lens be reduced but also the usual glass lens grinding dish can be employed with the glass lens.
The glass lens must have an n.sub.d of 1.523 for applying it to the usual glass lens grinding dish, but the modification of the n.sub.d value to such value is very difficult without reducing the ability of the glass to respond to light (hereinafter, light responding property) and the fading rate.
We, the inventors, have already proposed a reversible light sensitive glass having a high light responding property and the rapid fading rate, in U.S. Pat. application Ser. No. 232,667, filed on Mar. 8, 1972 which is based on the discovery that the coexistence of BaO and R.sub.2 O increases the light responding property and the rapid fading rate of the glass. If the amount of BaO for obtaining the maximum light responding property and fading rate is restricted, the increase of the value of the refractive index to 1.50 or more is impossible.
On the other hand, the commercially available reversible light sensitive glass contains a large amount of BaO to increase the refractive index value at 1.523 and is used in combination with PbO for the same purpose inspite of the adverse effects of PbO on the light responding property and fading rate.
Accordingly, the resultant glass exhibits a poor light responding property and especially a low fading rate. For example, in the commercial glass the restoration of the up to 90% of the initial transmission requires about 60 min.
Besides, most reversible light sensitive glasses are not completely durable to weathering, although weather resistance is required especially for a glass which is always exposed to the atmosphere.